Relationship between urinary calcium and calcium intake during calcitriol administration

Smothers, Ruth L.; Levine, Barton S.; Singer, Frederick R.; Bryce, Graeme F.; Mallon, John P.; Miller, O. Neal; Coburn, Jack W.
February 1986
Kidney International;Feb1986, Vol. 29 Issue 2, p578
Academic Journal
The hypercalciuria that occurs when 1.25 (OH)2D, (calcitriol) is given to humans with normal renal function depends on dietary Ca absorption and may also relate, in part, to enhanced bone resorption. To evaluate the relationship between urinary and dietary Ca during treatment with calcitriol, 12 metabolic balance studies were performed in normal volunteers ingesting a diet containing 350 mg/day of Ca, to which Ca gluconate was added. After 10 days on either 350 mg/day or 1550 mg/day of Ca. calcitriol, 0.5 μg every 12 hr. was given. Then diet Ca was changed in successive 5-day treatment periods from 350 to 650, 950 and 1550 mg/day (group A) or from 1550 to 950, 650 and 350 mg/day (group B). On the lowest diet Ca, urinary Ca was less than Ca intake during calcitriol treatment (group A. 220 ± 50 mg/day; group B, 247 ± 40). As diet Ca was changed during calcitriol treatment, urinary Ca correlated with diet Ca (r = 0.60) until diet Ca reached 950 mg/day. With calcitriol, serum iPTH fell by 18 to 25% (P < 0.01) and urinary hydroxyproline fell by 11 to 19% (P < 0.05 to 0.01). Baseline serum levels of 1,25(OH)2D were 47 ± 8 and 34 ± 5 pg/ml in group A and B. respectively, and the values increased to 51 ± 12 and 45 ± 7.4 pg/ml during treatment with calcitriol. Serum Ca from fasted subjects was not affected by calcitriol. but the mean postabsorptive serum Ca (noon) was increased by 0.35 mg/dl. Although urine Ca/creatinine from fasted subjects increased with calcitriol treatment, the values varied directly with the 24-hr urine Ca and inversely with serum iPTH levels. Thus, dietary Ca is the major determinant of urinary Ca during treatment with calcitriol, and the latter may decrease dietary Ca requirements. There was no evidence for an increased bone resorption. The reduction of hydroxyprotine excretion suggests that bone resorption was initially depressed, perhaps due to iPTH suppression. The data also suggest that urine Ca/creatinine after fasting for 12 hr is influenced by previous dietary Ca intake or intestinal Ca absorption, perhaps related to changing iPTH levels.


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